The conventional isolator does its job, if it is a field displacement isolator, by having the incoming field signal sent down the device, being guided by a metallic strip, and leaving the via the right port, with little or no attenuation. Referring now to FIG. 1, a conventional isolator 10 uses as a substrate 12a ferrite ceramic material. On a middle surface (not illustrated) of the ferrite substrate is a conductive metal strip (not illustrated), e.g. gold, typically of uniform width and relatively thin compared to the substrate 12 thickness. The conductive strip is what guides the signal, e.g. in the form of microwave energy, through isolator 10. Off to one side of the conductive strip is another strip (not illustrated) typically of a thin material, but unlike the conductive strip it is poorly conductive and very lossy. When the signal is inputted to a right port 14 and exiting on a left port on the side opposite the right port (not illustrated), the electromagnetic field distribution overlaps the lossy strip. The lossy strip is sampled by the signal, and attenuated, and as the energy flows through the device, it gets absorbed increasingly, so that as it approaches the left port 16, almost no signal amplitude remains. The electromagnetic field distribution is asymmetric, and that is why when the signal flows in the opposite direction, the fields do not see the lossy strip. Conventional isolators are described in F. J. Rosenbaum, Integrated Ferrimagnetic Devices, in Advances in Microwaves 8, 203 (Academic Press, New, 1974). J. D. Adam et al., IEEE Trans. Microwave Th. & Tech. 50, 721 (2002). M. E. Hines, IEEE Trans. Microwave Th. & Tech. 19, 442(1971). F. N. Bradley, Materials for Magnetic Functions (Hayden, N.Y. 1971). P. J. B. Clarricoats, Microwave Ferrites (Wiley, N.Y., 1961). R. F. Soohoo, Theory & Applications of Ferrites (Prentice-Hall, N.J., 1960). To summarize these references regarding the bias field, for principal axis static bias H0 field, asymmetry occurs in a direction normal to H0 and propagation direction kp. Edge guided microstrip isolator uses H0{circumflex over (x)}, earlier waveguide structure used H0ŷ.
Although conventional ferrite isolators work well, and are widely used in the various frequency bands throughout the microwave and millimeter wavelength regions, there are several severe drawbacks. One—they must employ static bias magnetic fields on the order of several thousand gauss (many tesla). To make these fields requires the use of either large magnets or large coils with circulating current, which are substantially larger than the other circuit elements and the device itself that actually propagates, controls and directs the signals. Two—both magnets and coil arrangements require what are termed magnetic circuits to direct the magnetic bias fields flow where desired in order to apply a static bias field to the ferrite material. This is because the effective action of the ferrite occurs when an external magnetic field lines up the precessing magnetic spin moments associated with the magnetic atoms in the ceramic material. Such isolator devices therefore rely on the spin precession effect found in ceramic ferrites. Three—the biasing magnets or coils add substantial additional weight and size to the device in the overall structure of conventional isolators. Magnetic circuits also add a substantial amount of weight and size. Four—finally, a substantial part of the cost of a conventional isolator is from the high quality low loss ferrite material, the entire magnetic circuit, and the magnets or coils.
There is, therefore, a need for an electronic isolator that is smaller and less expensive than existing conventional devices.